How do you revoke (or remove) a mail SSL certificate?

I have a StartCom SSL certificate on my emails, which is now showing as untrusted. I have created a new SSL certificate through InstantSSL and installed it and have removed the StartCom certificates. However. the mails still go out with the old StartCom certificate.

I don't know how to revoke the StartCom certificate and can't find anything on the system (Mail or KeyChain) which shows what certificate the mail is using.

??

OSX 10.10.5 Yosemite on MacBook Pro

Thanks for any pointers.